Bring a large pot of water to a rolling boil. Salt generously until it tastes like the ocean. Cook fusilli until al dente following package directions, about 8 to 10 minutes. Before draining, scoop out 1/2 cup of starchy pasta water and set aside. Drain pasta.

Pat chicken cubes completely dry with paper towels. Season with salt, pepper, and 1/2 tsp Italian seasoning. Heat 1 tbsp butter in a large skillet over medium-high heat until shimmering. Sear chicken in a single layer for 3 to 4 minutes per side until deep golden brown and cooked through to 165F internal temperature. Transfer to a plate, reserving all pan juices.

Lower heat to medium. Add remaining 2 tbsp butter and let melt. Add minced garlic and red pepper flakes if using. Stir constantly for 30 to 60 seconds until fragrant. Do not let garlic burn as it will make the sauce bitter.

Pour in chicken broth and scrape up all the caramelized bits from the bottom of the pan. Stir in heavy cream and remaining 1/2 tsp Italian seasoning. Simmer gently for 2 to 3 minutes until the sauce begins to thicken.

Reduce heat to low. Gradually sprinkle in Parmesan cheese in small additions, stirring continuously after each one until fully melted and the sauce is smooth and coats the back of a spoon.

Return chicken and all resting juices to the skillet. Add drained pasta and toss to coat. If sauce is too thick, add reserved pasta water a splash at a time until glossy. Taste and adjust seasoning, top with fresh parsley, and serve immediately.